Optical 2D Coherent Spectroscopy of Ultrafast Carrier Dynamics in Methylammonium Lead Iodide Perovskites

Hybrid organic-inorganic perovskite materials, especially the organometal halide perovskites (AMX3), have attracted tremendous attention due to their potential for solar cells. The perovskites also possess desired properties that make them promising candidates for applications in field transistor, light-emitting diode, and laser. To realize their potential applications, it is essential to have a fundamental understanding of optical properties and carrier dynamics of perovskite materials.

Here we use optical two-dimensional coherent spectroscopy (2DCS) to probe carrier dynamics in a MAPbI₃ film. Compared to transient absorption spectroscopy, optical 2DCS excels in probing complex dynamics and coupling and has been used to study complex systems including perovskites. We used optical 2DCS to study [1] the ultrafast dynamics and coupling of the dual emission features of the orthorhombic phase in MAPbI₃ at a temperature of 115 K. The 2D spectra revealed the redistribution of photoexcited hot carriers into the two emission resonances, their decay dynamics, and coherent phonon interactions.

In the current study [2], we obtained 2D spectra of photoexcited carriers in a MAPbI₃ film at room temperature. A set of 2D spectra at different waiting times were obtained. At short waiting times, the spectra feature two resonances with emission energies at about 1562.63 meV and 1605.66 meV. As the waiting time increases, the spectrum becomes just one resonance. The off-diagonal peak at short waiting times can be contributed by a short-lived transient excitonic state, while the diagonal peak corresponds to the free carriers. The spectra reveal that the photoexcited excitons form almost instantaneously with the pulse duration. After the formation of excitons, the excitons dissociated within only 300 fs. The ultrafast dynamics of the dissociation process of excitons are captured in the spectra. After dissociation, free carriers can persist for over 1000 ps, however, start to lose correlation at longer delay time as evident by the spectral diffusion in 2D spectra. The coherent phonon interaction was also observed.